Conventional fluid-operated percussion drill tools comprise an external cylinder or outer wear sleeve, within which is mounted an inner cylinder which in turn engages with a backhead assembly. A sliding reciprocating piston cooperates with the inner cylinder and backhead assembly, which when air pressure is supplied through the backhead assembly, acts with a percussive effect on a drill bit retained within a chuck on the outer wear sleeve.
In conventional arrangements, the inner cylinder is effectively suspended within the outer wear sleeve by means of a compressible retaining ring, such as a circlip, which has to be slid up the inner cylinder so as to seat against a shoulder or lip at one end thereof, being compressed when the inner cylinder is dropped down within the outer wear sleeve, and then expanding outwardly into a groove or shoulder formed on the inner diameter of the outer wear sleeve with a snap action. When in this position, the circlip seats within the groove and abuts against the lip of the inner cylinder, by which the inner cylinder is suspended within the outer wear sleeve. Compression of the circlip requires considerable force during this assembly operation.
A disadvantage with the circlip mounting system is that the inner cylinder may move during operation because of vibration, and the lip may become deformed, and unevenly worn due to wear between the lip and the circlip. The circlip is not a complete ring and includes a gap, and there is therefore no wear on the lip above the circlip gap. The hardness of the steel from which the inner cylinder and the circlip are made may not be the same, which also contributes to wear between these two components.
However, if the inner cylinder is removed from the outer wear sleeve for maintenance or inspection purposes, and then reassembled in the manner described above, the circlip is most likely to snap into place in the groove in the outer wear sleeve in a different position, i.e. the circlip gap is likely to be in a different position, so that wear between the circlip and the lip of the inner cylinder will be concentrated on a different portion of the lip. With continued wear, the result is that a slightly raised portion of the lip or lump will develop, and the presence of one or more lumps around the circumference of the lip caused by this uneven wear will result in poorer engagement between the inner cylinder and the outer wear sleeve, and even a skew or off-center between the two cylindrical components, which will rapidly increase wear between the components and shorten the life of the downhole drill.
The present invention seeks to provide a solution to the general problem of awkwardness and difficulty of assembly and reassembly of parts using a conventional circlip mounting, and furthermore to address the problems caused by uneven wear between the conventional circlip and the lip of the inner cylinder.